Frost accumulated on vegetation at the side of the Wales coast path.
Photograph: John Gilbey

The amount of ice on the narrow footpath came as a surprise. Hidden from the sun on the south side of the valley, it had probably accumulated over a number of days – along with the layered, crusted frost on the nearby vegetation. Few people seemed to have walked this stretch of the Wales coast path recently; a fox crossing the track ahead and a briefly perched buzzard both seemed shocked to see me.

As I reached the footbridge above the beach at Wallog, I folded away my heavily used Ordnance Survey map and consulted its nautical equivalent. While OS maps provide only scant information about the space beyond the coast, admiralty charts give detailed data on the underwater landscape – in this case the geological oddity that is Sarn Wallog.

Glacial in origin, this feature is one of several similarly narrow lines of stone that reach westwards from the coast of Wales. Made up of water-rounded cobbles, graded in size by the elements and the passage of time, they look beguilingly like the work of our ancestors – a pervasive theme in the rich historic literature of Wales.

Cliffs and teeming clefts in the coastal landscape

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Sarn Wallog and its close companion Sarn Cynfelyn reach out more than 10km towards Ireland, concealed in places by only a metre or so of seawater at certain states of tide, the seaward extremity being marked by the Patches buoy.

The sarn forms a natural breakwater, and as the tide fell the extent of this natural “causeway” below the sea was visible from its influence on the surface of the water. To the south small wavelets fussed confusedly, but north of the sarn a darker stillness prevailed – drawing the line of the feature across Cardigan bay.

As though for scale, three people, heavily muffled against the cold, walked out to the seaward end of the exposed bank and began taking photos, making it look more like a causeway than ever. The already low sun edged behind thickening cloud and the chill of the wind became profound, my thoughts turning to tea and a crackling fireplace.